Optical networks are an increasingly important part of today's communication networks. Optical networks use optical fibers to enable faster, more accurate, communication. An optical fiber (or fibre) is a glass, plastic, or other transparent fiber that carries light along its length. Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher data rates (a.k.a. “bandwidth”) than other forms of communications. Fibers are used instead of metal wires because signals travel along fibers with less loss, and fibers are also immune to electromagnetic interference. Specially designed fibers are used for a variety of other applications, including sensors and fiber lasers.
Light is kept in the “core” of the optical fiber by total internal reflection. This causes the fiber to act as a waveguide. Fibers which support many propagation paths or transverse modes are called multi-mode fibers (MMF). Fibers which can only support a single mode are called single-mode fibers (SMF). Multi-mode fibers generally have a larger core diameter, and are used for short-distance communication links and for applications where high power must be transmitted. Single-mode fibers are used for most communication links longer than 200 meters.
Differing propagation speeds for different wavelengths of light leads to a problem in optical networks called dispersion. Dispersion causes pulses to spread in optical fibers, degrading signals over long distances and possibly introducing errors. Dispersion is sometimes called chromatic dispersion to emphasize its wavelength-dependent nature.
Optical fibers, like any other material, have a refractive index. The refractive index (or index of refraction) of a medium is a measure of how much the speed of light is reduced inside the medium. The larger the index of refraction, the more slowly light travels in that medium. For example, typical soda-lime glass has a refractive index of 1.5, which means that in soda-lime glass, light travels at 1/1.5=0.67 times the speed of light in a vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays may change direction when they cross the interface from one material to another material, an effect that is used in lenses. Second, light reflects wholly or partially from surfaces that have a refractive index different from that of their surroundings.
An optical fiber consists of a core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. This produces total internal reflection, which keeps the optical signal within the core of the fiber. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber. A typical value for the cladding of an optical fiber can be 1.46. A typical value for the core of an optical fiber can be 1.48.
In general, the refractive index is some function of the frequency f of the light, thus n=n(f), or alternatively, with respect to the wave's wavelength n=n(λ). I.e., in general, the refractive index of a material is not a set value for all wavelengths. Instead, the refractive index varies according to the wavelength of the light transmitted. Therefore, in general, light of different wavelengths may propagate at different speeds through an optical fiber. The phase velocity, or propagation velocity, v, of an electromagnetic wave in a given, uniform, medium is given by
  v  =      c    n  where c is the speed of light in a vacuum and n is the refractive index of the medium.
There are generally two sources of dispersion: material dispersion and waveguide dispersion. Material dispersion comes from a frequency-dependent response of a material to waves. For example, material dispersion leads to undesired chromatic aberration in a lens or the separation of colors in a prism. Waveguide dispersion occurs when the speed of a wave in a waveguide (such as an optical fiber) depends on its frequency for geometric reasons, independent of any frequency dependence of the materials from which the waveguide is constructed. More generally, waveguide dispersion can occur for waves propagating through any inhomogeneous structure (e.g. a photonic crystal), whether or not the waves are confined to some region. In general, both types of dispersion may be present, although they are not strictly additive.
The combination of material dispersion and waveguide dispersion can lead to signal degradation in optical fibers, because the varying delay in arrival time between different components of a signal “smears out” the signal in time. Signal degradation, in turn, can introduce errors into the signal.
One method of removing dispersion involves converting the optical signal to an electric signal. The electric signal is then corrected and converted back to an optical signal. However, this process is time consuming relative to the speed at which optical networks operate. Therefore, there is a need for devices that can perform dispersion correction on the optical signal itself.